A variety of conventional techniques for assisting those with high-frequency hearing loss involve lowering high-frequency speech information into lower-frequency regions. Common to all these techniques is a positive rank scaling of frequency, such that the ordering, from lowest to highest, of frequency components in the higher frequency region of the input that is to be moved to the lower-frequency region is maintained in the output (after lowering into the lower-frequency region).
For example, FIG. 1 illustrates some of these conventional techniques involving positive rank scaling. In particular, FIG. 1(a) illustrates an example frequency range 101 of sound, a high-frequency region 104, which is illustrated as inaudible to a theoretical hearing-impaired person, and a low-frequency region 102, which is illustrated as audible to the theoretical hearing-impaired person.
FIGS. 1(b)-(e) illustrate conventional techniques for assisting the hearing-impaired person hear the high-frequency inaudible region 104 by shifting sounds within the high-frequency inaudible region 104 to the lower-frequency audible region 102. In this regard, it can be seen in FIG. 1 that in principle none of these techniques produce sound within the inaudible region 104 and, consequently, each of these techniques is illustrated within the audible region 102. In addition, each of these techniques involves positive rank scaling, as discussed below.
To elaborate, FIG. 1(b) illustrates a conventional linear frequency compression technique 106. This technique searches for sound within the high-frequency inaudible region 104. If sound is detected within the high-frequency inaudible region 104, such as during time period 106b, the entire frequency range 101 is linearly compressed so that it fits within the lower frequency audible region 102. This linear compression is illustrated in FIG. 1(b) with the uniform zigzag line 107, which has consistent internal angles. Such linear compression exhibits positive rank scaling by maintaining, in the post-compression output, the ordering of the frequencies present in the input sound pre-compression. When the hearing aid device does not detect sound within the high-frequency inaudible region 104, no compression of the input sound occurs, such as during time periods 106a and 106c. 
FIG. 1(c) illustrates a conventional linear frequency transposition technique 108. This technique continually searches for an intense spectral peak in a limited frequency range called a “source region.” This “source region” is within the inaudible region 104. When an intense spectral peak is detected within the “source region”, a frequency range including the intense spectral peak is transposed one octave below into the audible region 102 as illustrated by the transposed regions 108a-c. Each of these transposed regions 108a-c exhibits positive rank scaling, in that the ordered relationship between frequencies in the transposed input sound is maintained in the transposed regions of the output sound.
FIG. 1(d) illustrates a conventional spectral feature translation technique 110. This technique searches for spectral features in the high-frequency inaudible region 104 that are characteristic of speech. If it is detected that there is a likelihood that speech information exists in the high-frequency inaudible region 104, such as during time period 110b, a frequency range including the suspected high-frequency speech information is transposed or translated on an octave scale into the lower-frequency audible range 102. This translated frequency range is illustrated in FIG. 1(d) with the box 110d and exhibits positive rank scaling, in that the ordered relationship between frequencies in the transposed input sound is maintained in the transposed region of the output sound. If it is detected that there is not a likelihood that speech information exists in the high-frequency inaudible region 104, such as during time periods 110a and 110c, no translation of the input sound occurs.
FIG. 1(e) illustrates a conventional nonlinear frequency compression technique. This technique compresses frequencies above a start frequency 112a non-linearly over time to emphasize certain frequencies or ranges, while maintaining positive rank scaling in the compressed region. This non-linear compression is illustrated by the non-uniform zigzag lines having differing internal angles shown in FIG. 1(e), one of which is called out as reference 112b. The frequencies below the start frequency 112a are not compressed.
While the conventional techniques of FIG. 1, and other conventional techniques involving positive rank scaling in shifted frequencies, assist hearing-impaired individuals in hearing otherwise inaudible sounds, there is a need in the art for further improvement of hearing aid devices.